S-adenosylhomocysteine hydrolase plays a critical role in regulating AdoMet-dependent methylations in eukaryotic cells by regulating the ratio of AdoMet/AdoHcy. Several approaches are being used to determine the structure and function of this enzyme. 1) Structure Determination: The enzyme has been purified to homogeneity from rat liver. Conformational changes for active and inactive forms of the enzyme have been examined by fluorescence and circular dichroism. Peptide fragments of the protein have been isolated and partial amino acid sequences determined. Oligonucleotide probes from the peptide sequences and antibodies are being used to screen a cDNA library from rat liver to obtain the nucleic acid sequence. 2) Ligand Binding and Kinetic Properties: The role of NAD, nucleotide, and cAMP binding in regulating the catalytic activity has been studied. A large number of adenosine and adenosylhomocysteine analogs have been examined for their ability to function as inhibitors and/or substrates of the enzyme. The two most interesting compounds studied are 3-deazaadenosine and 3-deazaaristeromycin. Both compounds inhibit the enzyme, but only 3-deazaadenosine is a substrate. 3) Inhibition of AdoHcy Hydrolase in vivo: Analogs of adenosylhomocysteine synthesized by this enzyme in vivo can form very potent and specific inhibitors of transmethylation reactions, and provide useful probes for studies on the role of specific methylation reactions in biological functions. These analogs have a wide range of biological activities, including antiviral activity against several RNA and DNA viruses, inhibition of leukocyte chemotaxis, and stimulation of cell differentiation in myoblast and erythroid cell lines.